Although balloons have been used for useful applications for over 200 years, for over 160 of these 200 years, balloons were made from rubber and fabric, and were too heavy to escape the lower atmosphere. It is only in recent years that the development of plastic films has made high altitude balloons possible. These films, first developed in the 1940's, have permitted the manufacture of balloons that can reach the stratosphere and near space.
High altitude balloons are designed to carry a payload into altitudes as high as 120,000 feet or higher. These balloons stay aloft in the air by being filled with lighter than air lift gases. Typical uses of high altitude balloons are communications and observation platforms and geophysical and astrophysical research.
Many high altitude balloons are "low pressure" balloons and are vented so that the lift gas inside the balloon may escape. This helps preserve the integrity of the balloon material, but encourages the balloon to change volume, which causes its altitude to change. Ballast is used to help maintain a constant altitude. However, venting and ballast are effective only to the extent that low pressure balloons are useful for short duration flights in the order of several days.
High pressure balloons are another approach to maintaining constant volume. A problem with high pressure balloons, however, is that the balloon must not only withstand stresses due to the payload, but also those produced by pressurization. These stresses affect the characteristics of the material used to construct the balloon, such as by stretching or deterioration, and cause the volume of the balloon to change or cause destruction of the balloon.
One characteristic of potential balloon materials, that determines their suitability for high pressure balloons is known as "creep". Creep is a mechanical behavior of materials that continue to strain with time when subjected to a constant stress even at a constant temperature. More technically, creep is the time-dependent portion of strain. For creep-susceptible materials, increasing either stress or temperature increases creep. When creep is present, material failure may occur at stresses or temperature that are below those present during short duration uses. Certain materials, notably nylon, have been rejected in the past due to their susceptibility to creep.
Creep is only one property of potential balloon materials that affects the success of the balloon. Also, good properties in one area often detract from the properties in another area. Thus, selection of materials is an important decision in the design process. A common material used for existing high pressure balloons is polyester, such as Mylar, which has good strength and modulus characteristics. Also used are layers of different plastic films, with each layer selected for certain desired properties.
A problem with previous attempts to maintain long duration flights of high pressure balloons is failure of the balloon material. These failures are attributable to a number of factors, especially including temperature extremes and high gas pressures. Furthermore, the failure rate increases as the payload and therefore the balloon size and pressure increase. The high failure rate of high altitude, high pressure balloons, combined with the expense of trial and error balloon testing, has led to a reluctance to experiment with new materials. A need exists for a balloon that can be especially designed to withstand the pressures and temperatures of high altitude, long duration flights.